(225g) Modeling the Sintering of Water Soluble Amorphous and Crystalline Particles

Plasticization of water soluble amorphous carbohydrates by moisture sorption is a ubiquitous phenomenon influencing the stability and functionality of pharmaceutical excipients and food products. Indeed, because of its very low molecular weight, water enhances molecular mobility and is thus a highly efficient plasticizer. As a consequence, the glass transition temperature of an amorphous matrix decreases strongly as a function of the water content. Thus when a glassy carbohydrate is placed at high relative humidity, its glass transition temperature may reach a lower value than the one of the product itself. A dramatic decrease of the viscosity of the material happens, leading to the development of viscous flow. Viscous flow is responsible for the formation of viscous bridges between the particles also called sintering. Amorphous water-soluble particles are sintering together depending on time, temperature, stress and water content. In case the particles are in equilibrium with the surrounding air the sinter kinetics can be predicted by combining the known sinter equations with the theory of glass transition. However in practice the particles are often not in their equilibrium state and moisture migration and heat transfer are influencing the sintering kinetics. Based on the sinter equation from Rumpf et al.[1] and the theory of glass transition the growth of the sinter bridge between two amorphous particles was modeled as a function of time, viscosity and surface tension. In addition amorphous maltodextrine particles were exposed under a microscope in controlled climate conditions. The development of the sinter-bridge between two particles was monitored depending on time, temperature and relative humidity of the surrounding air. The obtained time dependent distribution of the sinter-bridge diameter was compared with the calculated bridge diameter using Rumpf's model [1]. The same experimental approach was used to measure bridge formation between crystalline (NaCl) and amorphous (maltodextrin DE21) particles. It appears from this study that Rumpf et al.'s model [1] well describes the sintering kinetics of amorphous particles in a qualitative way. However, its use for a quantitative prediction is limited. Indeed, the experimental sintering kinetics of amorphous particles is slower than the theoretical calculations. Concerning the bridge formation between crystalline salt and amorphous maltodextrin components, the measured kinetics is faster than predicted by the theory.

1 Rumpf, H., Sommer, K., and Steier, K. Mechanismen der Haftkraftverstärkung bei der Partikelhaftung durch plastisches Verformen, Sintern und viskoelastisches Fließen. Chem.-Ing.-Tech. 48 (4), 300 (1976).